This invention concerns a method of and apparatus for processing colour television signals.
Occasions arise when processing colour television signals, e.g. of PAL or N.T.S.C. type, where it is desired to provide in conjunction with any given part of the signal, (the "current" signal), a signal derived from an earlier part of the television signal which is likely to be similar in content and is similar in form to the current signal. Such an additional or supplementary signal can be used to replace the current signal if the latter is known to contain errors, or to provide a reference for the differential coding system, or as described in U.S. Pat. No. 4,058,836 (Drewery et al) to enable a determination of the magnitude of the difference in the signals. More particularly, said earlier patent describes an arrangement for reducing the effect of noise in the signal, in which from the input signal for the current field or picture scan is subtracted the output for the preceding scan to provide a difference signal. Low amplitude portions of this signal are assumed to represent noise and are attenuated, while high amplitude portions are assumed to represent a change in picture detail and are not. The thus-modified difference signal is then added again to the output signal for the preceding scan to provide an output signal for the current scan in which the effects of noise have been subjectively reduced.
When the system is applied directly to an encoded composite colour television signal, it is necessary to ensure that the two signals used to form by subtraction the difference signal are inherently of the same form. Otherwise the difference signal will not properly represent changes in picture content but will also include components due to differences in the form of the signals.
With a view to overcoming this problem said earlier patent 4,058,836 shows in FIG. 7 of the drawings a form of a predictor which can be used to modify the signal from the preceding scan to convert it into the same form as the signal of the current scan. Such disclosure represents the closest prior art known to the Applicant. In particular, this predictor separates the luminance and chrominance components of the composite PAL signal, changes the phase of the chrominance component, and then recombines it with the luminance component.
Such a procedure is complex and liable to lead to degradation if not carried out with great accuracy.